1. Field of the Invention
This invention generally relates to an improvement of an overlock sewing machine. More particularly, the invention relates to an overlock sewing machine in which a mode of stitch formations, such as over-edge stitching, double chain stitching or the like, may be easily converted to each other by means of a simple stitch change-over operation.
2. Description of the Prior Art
A sewing machine is known which performs functions of both "chain stitching" and "over-edge stitching".
A conventional one-needle, three-thread overlock sewing machine for providing an over-edge stitch is disclosed in Japanese Patent Public Disclosure (KOKAI) No. 62-16785. The machine disclosed therein includes means for breaking off interlock relation between an upper looper and a main shaft, means for causing an oscillating shaft to which the lower looper is fixed to be reciprocally moved in the direction opposite to the direction in which material or cloth is fed, and means for releasing the shaft reciprocating means.
A conventional double chain-stitch machine is also disclosed in Japanese Utility Model Public Disclosure (KOKAI) No. 62-143483. This machine is designed to form a double chain stitch by means of a cooperative operation between a needle and a looper shaft which is reciprocal in a right-hand-left-hand direction and backward-rearward direction (laterally and longitudinally). This machine is so designed that a back and forth stroke of a looper shaft may be adjusted by adjusting a degree of inclination of an adjustment box of the machine.
The machine disclosed in Japanese Patent Public Disclosure No. 62-16785 is designed so as to perform both "over-edge stitching" and "double chain stitching" by incorporating into a conventional mechanism a separate and additional mechanism for actuating one-needle and three-threads. Thus, the mechanism of such a machine is essentially complicated and has a tendency to mechanical problems. As the machine is designed to work with one needle and three threads, it is incapable of using two needles. If it is desired to use two needles in the machine, a right-hand needle must be newly incorporated thereinto. Also, the width of the needle would become wider when the right-hand needle is caught by a looper. Accordingly, it is not possible to properly catch a thread at an appropriate timing. If it is necessary to adjust a level of the needle so as to appropriately catch a thread, a cloth to be sewn would be fed forward by a feed dog before the needle is withdrawn from the cloth or material, thus causing a problem of sidewise movement of the needle.
The machine disclosed in Japanese Utility Model Public Disclosure No. 62-143483 is so designed that a stroke of reciprocal movement of a looper shaft is adjusted by an adjustment box, whereby it is difficult to set a backward or forward feed amount at zero. Accordingly, it is disadvantageous in that a position of needle catch (clearance between the needle and looper) could not be maintained so as to be constant upon a change-over operation between "over-edge stitching" and "double chain stitching". The machine also has a disadvantage in that an amount of feed in a forward or rearward direction cannot be regulated easily.